Developmental commitments in Drosophila appear to arise sequentially, and result in the clonal isolation of neighboring populations of cells which have adopted diverse commitments. From the fate map location of such neighboring groups of cells, we infer that lines of clone restriction or compartmental boundaries should form sequentially and subdivide the blastoderm stage embryo. We have analyzed such lines using X-ray induced mitotic recombination to mark clones. We propose to use the the mit mutant, in which daughters of homozygous females lose one or the other X chromosomes in the first several cleavage divisions. Large clones, from first or second cleavage loss of the X, should not be contained in any compartment. Successively smaller clones should be trapped in a successively finer grid of compartmental boundaries, thus revealing a sequence of boundary formation. We plan to begin a long term analysis tesing whether the clone restriction boundaries we have seen on the blastoderm actually reflect commitment of cells to alternative fates, by transplanting genetically marked cells between different areas of the balstoderm, at controlled stages of development. Whether or not the compartment hypothesis is correct, we should be able to study the fate of single blastoderm cells.